Well Log Interpretation: Petrophysical Techniques

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Well Log InterpretationPetrophysical Techniques
Earth Environmental Science University of
Texas at Arlington
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Interpretation Techniques
Most of the Interpretation techniques have
already been introduced so this chapter is a
summary and in introduction to the use of facies
maps based on well loging.
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Interpretation TechniquesLithology

• The following logs and cross plots are used and
  modern logging software often computes and
  displays the results automatically
• Gamma Ray
• Neutron-Density
• Neutron-Sonic
• Density-Sonic
• M-N
• rmaa Dtmaa
• rmaa-Umaa

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Interpretation Techniques Lithology
The lithology cross plots may give conflicting
results especially when the sonic log is used in
carbonates with vuggy porosity. The sonic log
does not see widely spaced holes, so it
underestimates porosity and the Dtmaa variable,
and overestimates the M variable when vugs are
present.
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Interpretation Techniques Lithology
Example Lithology Logs
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Interpretation Techniques Lithology
Example Lithology Logs
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Interpretation Techniques
The previous examples show significant disparity
in the lower interval, so vuggy porosity in
carbonates is probably present. In those
circumstances, the neutron-density and rmaa -
Umaa data are the most reliable.
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Interpretation Techniques Clean Zones (shale
free)
Both the SP and Gamma Ray log can be used to map
clean sediments, which generally have higher
permeability and porosity.
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Interpretation Techniques Clean Zones (shale
free)
The SP response is depressed by shale, thin beds
and hydrocarbons. SP alpha mapping can be used to
determine clean zones, assuming a lower response
is produced only by shale.
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Interpretation Techniques Clean Zones (shale
free)
The first step is to compute the Static SP (SSP),
which is the ideal SP response for clean, thick
rocks.
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Interpretation Techniques Clean Zones (shale
free)
Using the shale baseline of the SP log, plot a
line showing the value of SSP. In the example
SSP-130 left dashed line on log.
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Interpretation Techniques Clean Zones (shale
free)
Pick some (alpha) of the SSP to be the shale
cutoff and draw a line on the log (the example
has two alpha values, 75 and 50, or -98 and -65
respectively).
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Interpretation Techniques Clean Zones (shale
free)
Measure the thickness of the clean interval
where the SP curve cuts the alpha line (either
27 for the 75 alpha or 45 for the 50 alpha)
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Interpretation Techniques Clean Zones (shale
free)
Measure the alpha thickness in all the wells and
contour. The idea is to look for trends to drill
additional production wells.
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Interpretation Techniques Clean Zones (shale
free)
Of course thin beds and hydrocarbons depress the
SP log so the thick alphas could be where
hydrocarbons are absent and thin alphas where
they are present.
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Interpretation Techniques Clean Zones (shale
free)
Gamma Ray logs are better than SP logs for shale
determination. First determine GRmax (100 shale)
and GRmin (0 shale). From formulas in Ch. 3 or
using Figure 3.2, determine IGR for Vsh5 (or
whatever Vsh cutoff value you want). In this case
IGR5 0.10
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Interpretation Techniques Clean Zones (shale
free)
If IGR5 0.10, then
In the example, GR5 40 API units.
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Interpretation Techniques Clean Zones (shale
free)
In the example a line is drawn at 40 API units
and the thickness of the interval where the GR
log reads less than 40 is the thickness of the
clean interval in this case about 44.
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Interpretation Techniques Clean Zones (shale
free)
The clean thicknesses from a number of wells
can then be contoured to determine the trend of
the most promising zone.
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Interpretation Techniques Create your own
crossplot
If petrographic data is available from cores and
cuttings, and sedimentary facies can be
identified in several wells, then the log
response to those various facies can be
identified using various cross plots.
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Interpretation Techniques Create your own
crossplot
In the example, the various carbonate facies
determined from petrographic data plot in
specific regions on a Rt-FN crossplot. Zones in
wells without petrographic data can be assigned a
facies from the logs.
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Interpretation Techniques Create your own
crossplot
Facies maps can be made from the log data to
determine where additional wells should be
drilled (or not).